Nitrogen, Sulfur Co-Doped Hierarchically Porous Carbon as a Metal-Free Electrocatalyst for Oxygen Reduction and Carbon Dioxide Reduction Reaction

Recent Advances in the Design and Application of Asymmetric Carbon-Based Materials

Asymmetric carbon-based materials (ACBMs) have received significant attention in scientific research due to their unique structures and properties. Through the introduction of heterogeneous atoms and the construction of asymmetric ordered/disordered structures, ACBMs are optimized in terms of electrical conductivity, pore structure, and chemical composition and exhibit multiple properties such as hydrophilicity, hydrophobicity, optical characteristics, and magnetic behavior. Here, the recent research progress of ACBMs is reviewed, focusing on the potential of these materials for electrochemical, catalysis, and biomedical applications and their unique advantages over conventional symmetric carbon-based materials. Meanwhile, a variety of construction strategies of asymmetric structures, including template method, nanoemulsion assembly method, and self-assembly method, are described in detail. In addition, the contradictions between material synthesis and application are pointed out, such as the limitations of synthesis methods and morphology modulation means, as well as the trade-off between property improvement and production costs. Finally, the future development path of ACBMs is envisioned, emphasizing the importance of the close integration of theory and practice, and looking forward to promoting the research and development of a new generation of high-performance materials through the in-depth understanding of the design principles and action mechanisms of ACBMs.

Decoding Dual-Functionality in N-doped Defective Carbon: Unveiling Active Sites for Bifunctional Oxygen Electrocatalysis

Oxygen electrocatalysis plays a pivotal role in energy conversion and storage technologies. The precise identification of active sites for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for developing an efficient bifunctional electrocatalyst. However, this remains a challenging endeavor. Here, it is demonstrated that metal-free N-doped defective carbon material derived from triazene derivative exhibits excellent bifunctional activity, achieving a notable ΔE value of 0.72 V. Through comprehensive X-ray photoelectron spectroscopy and Raman spectroscopic analyses, the active sites responsible for oxygen electrocatalysis are elucidated, resolving a long-standing issue. Specifically, pyridinic-N sites are crucial for ORR, while graphitic-N are good for OER. A predictive model utilizing π-electron descriptors further aids in identifying these sites, with theoretical insights aligning with experimental results. Additionally, in situ ATR-FTIR spectroscopy provides clarity on reaction intermediates for both reactions. This research paves the way for developing metal-free, site-specific electrocatalysts for practical applications in energy technologies.

Tailoring vinegar residue-derived all-carbon electrodes for efficient electrocatalytic carbon dioxide reduction to formate through heteroatom doping and defect enrichment

Electrocatalytic carbon dioxide reduction (ECO2R) to formate is the most technically and economically feasible approach to achieve electrochemical CO2 value addition. Here, a few-layer graphene is prepared from vinegar residue. Then a series of heteroatom-doped vertical graphene electrodes (X-rGO, X=P/S/N/B/, NS/NP/NB, NSP/NSB/NPB/NSPB) are prepared. The NS-rGO has improved ECO2R to formate selectivity (Faraday Efficiency (FEHCOO-) = 78.7 %) thanks to the synergistic effect between N and S. Carbon quantum dots (CQDs) are introduced into the electrode, the doped heteroatoms are further removed by high-temperature to form the defects-rich electrode (NS-CQDs-rGO-1100), which has better catalytic performance (FEHCOO-=90 %, stability over 10 h) with electrochemical double layer capacitance of 12.5 mF cm-2. The intrinsic effect of heteroatom doping and defects on the ECO2R activity of the electrodes are explored by density functional theory calculation. This work broadens the field of preparation of graphene and opens the door to the development of cost-effective electrocatalysts for efficient ECO2R.

Self-Powered Electrochemical CO2 Conversion Enabled by a Multifunctional Carbon-Based Electrocatalyst and a Rechargeable Zn-Air Battery

Multifunctional electrocatalysts are required for diverse clean energy-related technologies (e.g., electrochemical CO2 reduction reaction (CO2RR) and metal-air batteries). Herein, a nitrogen and fluorine co-doped carbon nanotube (NFCNT) is reported to simultaneously achieve multifunctional catalytic activities for CO2RR, oxygen reduction reaction (ORR), and oxygen evolution reaction (OER). Theoretical calculations reveal that the superior multifunctional catalytic activities of NFCNT are attributed to the synergistic effect of nitrogen and fluorine co-doping to induce charge redistribution and decrease the energy barrier of rate-determining step for different electrocatalytic reactions. Furthermore, the rechargeable Zn-air battery (ZAB) with NFCNT electrode delivers a high peak power density of 230 mW cm-2 and superior durability over 100 cycles, outperforming the ZAB with Pt/C+RuO2 based electrodes. More importantly, a self-driven CO2 electrolysis unit powered by the as-assembled ZABs is developed, which achieves 80% CO Faraday efficiency and 60% total energy efficiency. This work provides a new insight into the exploration of highly efficient multifunctional carbon-based electrocatalysts for novel energy-related applications.

Nanostructure Engineering of Cathode Layers in Proton Exchange Membrane Fuel Cells: From Catalysts to Membrane Electrode Assembly

The membrane electrode assembly (MEA) is the core component of proton exchange membrane fuel cells (PEMFCs), which is the place where the reaction occurrence, the multiphase material transfer and the energy conversion, and the development of MEA with high activity and long stability are crucial for the practical application of PEMFCs. Currently, efforts are devoted to developing the regulation of MEA nanostructure engineering, which is believed to have advantages in improving catalyst utilization, maximizing three-phase boundaries, enhancing mass transport, and improving operational stability. This work reviews recent research progress on platinum group metal (PGM) and PGM-free catalysts with multidimensional nanostructures, catalyst layers (CLs), and nano-MEAs for PEMFCs, emphasizing the importance of structure-function relationships, aiming to guide the further development of the performance for PEMFCs. Then the design strategy of the MEA interface is summarized systematically. In addition, the application of in situ and operational characterization techniques to adequately identify current density distributions, hot spots, and water management visualization of MEAs is also discussed. Finally, the limitations of nanostructured MEA research are discussed and future promising research directions are proposed. This paper aims to provide valuable insights into the fundamental science and technical engineering of efficient MEA interfaces for PEMFCs.

Hollow carbon-based materials for electrocatalytic and thermocatalytic CO2 conversion

Electrocatalytic and thermocatalytic CO2 conversions provide promising routes to realize global carbon neutrality, and the development of corresponding advanced catalysts is important but challenging. Hollow-structured carbon (HSC) materials with striking features, including unique cavity structure, good permeability, large surface area, and readily functionalizable surface, are flexible platforms for designing high-performance catalysts. In this review, the topics range from the accurate design of HSC materials to specific electrocatalytic and thermocatalytic CO2 conversion applications, aiming to address the drawbacks of conventional catalysts, such as sluggish reaction kinetics, inadequate selectivity, and poor stability. Firstly, the synthetic methods of HSC, including the hard template route, soft template approach, and self-template strategy are summarized, with an evaluation of their characteristics and applicability. Subsequently, the functionalization strategies (nonmetal doping, metal single-atom anchoring, and metal nanoparticle modification) for HSC are comprehensively discussed. Lastly, the recent achievements of intriguing HSC-based materials in electrocatalytic and thermocatalytic CO2 conversion applications are presented, with a particular focus on revealing the relationship between catalyst structure and activity. We anticipate that the review can provide some ideas for designing highly active and durable catalytic systems for CO2 valorization and beyond.

Engineering Electronic Structure of Nitrogen-Carbon Sites by sp3 -Hybridized Carbon and Incorporating Chlorine to Boost Oxygen Reduction Activity

Development of efficient and easy-to-prepare low-cost oxygen reaction electrocatalysts is essential for widespread application of rechargeable Zn-air batteries (ZABs). Herein, we mixed NaCl and ZIF-8 by simple physical milling and pyrolysis to obtain a metal-free porous electrocatalyst doped with Cl (mf-pClNC). The mf-pClNC electrocatalyst exhibits a good oxygen reduction reaction (ORR) activity (E1/2 =0.91 V vs. RHE) and high stability in alkaline electrolyte, exceeding most of the reported transition metal carbon-based electrocatalysts and being comparable to commercial Pt/C electrocatalysts. Likewise, the mf-pClNC electrocatalyst also shows state-of-the-art ORR activity and stability in acidic electrolyte. From experimental and theoretical calculations, the better ORR activity is most likely originated from the fact that the introduced Cl promotes the increase of sp3 -hybridized carbon, while the sp3 -hybridized carbon and Cl together modify the electronic structure of the N-adjacent carbons, as the active sites, while NaCl molten-salt etching provides abundant paths for the transport of electrons/protons. Furthermore, the liquid rechargeable ZAB using the mf-pClNC electrocatalyst as the cathode shows a fulfilling performance with a peak power density of 276.88 mW cm-2 . Flexible quasi-solid-state rechargeable ZAB constructed with the mf-pClNC electrocatalyst as the cathode exhibits an exciting performance both at low, high and room temperatures.

Recent Advances and Synergistic Effects of Non-Precious Carbon-Based Nanomaterials as ORR Electrocatalysts: A Review

The use of platinum-free (Pt) cathode electrocatalysts for oxygen reduction reactions (ORRs) has been significantly studied over the past decade, improving slow reaction mechanisms. For many significant energy conversion and storage technologies, including fuel cells and metal-air batteries, the ORR is a crucial process. These have motivated the development of highly active and long-lasting platinum-free electrocatalysts, which cost less than proton exchange membrane fuel cells (PEMFCs). Researchers have identified a novel, non-precious carbon-based electrocatalyst material as the most effective substitute for platinum (Pt) electrocatalysts. Rich sources, outstanding electrical conductivity, adaptable molecular structures, and environmental compatibility are just a few of its benefits. Additionally, the increased surface area and the simplicity of regulating its structure can significantly improve the electrocatalyst's reactive sites and mass transport. Other benefits include the use of heteroatoms and single or multiple metal atoms, which are capable of acting as extremely effective ORR electrocatalysts. The rapid innovations in non-precious carbon-based nanomaterials in the ORR electrocatalyst field are the main topics of this review. As a result, this review provides an overview of the basic ORR reaction and the mechanism of the active sites in non-precious carbon-based electrocatalysts. Further analysis of the development, performance, and evaluation of these systems is provided in more detail. Furthermore, the significance of doping is highlighted and discussed, which shows how researchers can enhance the properties of electrocatalysts. Finally, this review discusses the existing challenges and expectations for the development of highly efficient and inexpensive electrocatalysts that are linked to crucial technologies in this expanding field.

Zn-Induced Synthesis of Porous Fe-N,S-C Electrocatalyst with Iron-Based Active Sites Containing Sulfides, Oxides and Nitrides for Efficient Oxygen Reduction and Zinc-Air Batteries

There is an urgent need to design and synthesize non-noble metal electrocatalysts (NNMEs) for the replacement of platinum-based electrocatalysts to enhance the sluggish oxygen reduction reaction (ORR) for Zn-air batteries and fuel cells. Herein, Fe-N,S-C materials were fabricated through two steps: first, reprecipitating hemin by adjusting the pH and, then, decorating it with melamine and cysteine in the presence of Zn2+. The resulting Fe-N,S-C-950 (Zn) was prepared after pyrolysis at 950 °C. Using this method, abundant iron-based active species with good dispersion were obtained. The fabrication of more micropores in Fe-N,S-C-950 (Zn) plays a positive role in the improvement of ORR activity. On comparison, Fe-N,S-C-950 (Zn) outperforms Fe-N,S-C-950 and Fe-N-C-950 (Zn) with respect to the ORR due to its larger specific surface area, porous structure, multiple iron-based active sites and N- and S-doped C. Fe-N,S-C-950 (Zn) achieves outstanding ORR performances, including a half-wave potential (E1/2) of 0.844 V and 0.715 V versus a reversible hydrogen electrode (RHE) in 0.1 M KOH and 0.1 M HClO4 solution, respectively. In addition, Fe-N,S-C-950 (Zn) shows an outstanding Zn-air battery performance with an open-circuit voltage (OCV) of 1.450 V and a peak power density of 121.9 mW cm-2, which is higher than that of 20 wt% Pt/C. As a result, the as-prepared electrocatalyst in this work shows the development of the Zn-assisted strategy combined with the assembly of porphyrins as NNMEs for the enhancement of the ORR in both alkaline and acidic solutions.

Facilitating two-electron oxygen reduction with pyrrolic nitrogen sites for electrochemical hydrogen peroxide production

Electrocatalytic hydrogen peroxide (H₂O₂) production via the two-electron oxygen reduction reaction is a promising alternative to the energy-intensive and high-pollution anthraquinone oxidation process. However, developing advanced electrocatalysts with high H₂O₂ yield, selectivity, and durability is still challenging, because of the limited quantity and easy passivation of active sites on typical metal-containing catalysts, especially for the state-of-the-art single-atom ones. To address this, we report a graphene/mesoporous carbon composite for high-rate and high-efficiency 2e^- oxygen reduction catalysis. The coordination of pyrrolic-N sites -modulates the adsorption configuration of the *OOH species to provide a kinetically favorable pathway for H₂O₂ production. Consequently, the H₂O₂ yield approaches 30 mol g^-1 h^-1 with a Faradaic efficiency of 80% and excellent durability, yielding a high H₂O₂ concentration of 7.2 g L^-1. This strategy of manipulating the adsorption configuration of reactants with multiple non-metal active sites provides a strategy to design efficient and durable metal-free electrocatalyst for 2e^- oxygen reduction.

Designing a Hierarchical Porous Carbon with Optimized Nitrogen Doping for Efficient Oxygen Reduction Reaction

Nitrogen-doped carbon is considered one of the most promising oxygen reduction catalysts due to its low cost and high activity, however, it still falls short of Pt/C. In this study, we report a strategy for the preparation of highly reactive N-doped hierarchical porous carbon by primary pyrolysis using zinc acetate as a stand-alone zinc source and amino-rich reactants as carbon and nitrogen sources to introduce Zn-Nx structures into mesoporous structures generated by the hard template method using the strong coordination of zinc and amino groups. Benefited from the simultaneous optimization of the hierarchical porous structure and nitrogen-doping, the half-wave potential of Zn(OAc)2 -DCD/HPC is as high as 0.909 V vs. RHE, much better than that of commercial Pt/C catalysts (0.872 V vs. RHE). In addition, zinc-air batteries assembled with Zn(OAc)2 -DCD/HPC (Pmax =198 mW cm-2 ) as the cathode exhibit higher peak power density compared to Pt/C (Pmax =168 mW cm-2 ). This strategy might open up new opportunities for designing and developing highly active metal-free catalysts.

Carbon-Based Electrocatalysts for Acidic Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) is vital for clean and renewable energy technologies, which require no fossil fuel but catalysts. Platinum (Pt) is the best-known catalyst for ORR. However, its high cost and scarcity have severely hindered renewable energy devices (e.g., fuel cells) for large-scale applications. Recent breakthroughs in carbon-based metal-free electrochemical catalysts (C-MFECs) show great potential for earth-abundant carbon materials as low-cost metal-free electrocatalysts towards ORR in acidic media. This article provides a focused, but critical review on C-MFECs for ORR in acidic media with an emphasis on advances in the structure design and synthesis, fundamental understanding of the structure-property relationship and electrocatalytic mechanisms, and their applications in proton exchange membrane fuel cells. Current challenges and future perspectives in this emerging field are also discussed.

Three-dimensional porous high boron-nitrogen-doped carbon for the ultrasensitive electrochemical detection of trace heavy metals in food samples

Exposure to even trace amounts of Cd(II) and Pb(II) in food can have serious effects on the human body. Therefore, the development of novel electrochemical sensors that can accurately detect the different toxicity levels of heavy metal ions in food is of great significance. Based on the principle of green chemistry, we propose a new type of boron and nitrogen co-doped carbon (BCN) material derived from a metal-organic framework material and study its synthesis, characterization, and heavy-metal ion detection ability. Under the optimum conditions, the BCN-modified glassy carbon electrode was studied using square-wave anodic stripping voltammetry, which showed good electrochemical responses to Cd(II) and Pb(II), with sensitivities as low as 0.459 and 0.509 μA/μM cm², respectively. The sensor was successfully used to detect Cd(II) and Pb(II) in Beta vulgaris var. cicla L samples, which is consistent with the results obtained using inductively coupled plasma-mass spectrometry. It also has a strong selectivity for complex samples. This study provides a novel approach for the detection of heavy metal ions in food and greatly expands the application of heteroatom-doped metal-free carbon materials in detection platforms.

N, S dual doped mesoporous carbon assisted simultaneous electrochemical assay of emerging water contaminant hydroquinone and catechol

Heteroatom doped mesoporous carbon materials are promising catalysts for the electrochemical sensing application. Herein, we report highly efficient dual heteroatom-doped hexagonal mesoporous carbon (MC) derived from Santa Barbara Amorphous-15 (SBA-15) hard template for the detection of phenolic isomers. The synthesis involves dopamine hydrochloride (DA)/thiophene complex, which helps to attain perfectly retained N and S dual doped mesoporous carbon (NS-MC) framework. NS-MC exhibits higher surface area (951 m² g^-1) as well as higher pore volume (0.12 cm³ g^-1) with huge graphitic, pyridinic and thiophenic defective sites which facilitates the well-resolved simultaneous electrochemical detection of phenolic isomers hydroquinone (HQ) and catechol (CC). Our results demonstrate that as-synthesized NS-MC material had a LOD of 0.63 μM and 0.29 μM for HQ and CC, respectively. From the calibration curve, sensitivities of proposed sensor were found to be 9.44, 2.71 μA μM^-1 cm^-2 and 20.80, 10.02 μA μM^-1 cm^-2 for HQ and CC, respectively with good linear ranges of 10-45 μM and 45-115 μM for HQ; 2-16 μM and 16-40 μM for CC. The NS-MC modified electrode exhibited good selectivity over various possible interferences. The present investigation reveals that the proposed NS-MC material is a promising metal-free catalyst which boosted to electrochemically detect both HQ and CC, present in the municipal tap as well as natural river stream water samples.

Secondary-Heteroatom-Doping-Derived Synthesis of N, S Co-Doped Graphene Nanoribbons for Enhanced Oxygen Reduction Activity

The rareness and weak durability of Pt-based electrocatalysts for oxygen reduction reactions (ORRs) have hindered the large-scale application of fuel cells. Here, we developed an efficient metal-free catalyst consisting of N, S co-doped graphene nanoribbons (N, S-GNR-2s) for ORRs. GNRs were firstly synthesized via the chemical unzipping of carbon nanotubes, and then N, S co-doping was conducted using urea as the primary and sulfourea as the secondary heteroatom sources. The successful incorporation of nitrogen and sulfur was confirmed by elemental mapping analysis as well as X-ray photoelectron spectroscopy. Electrochemical testing revealed that N, S-GNR-2s exhibited an E_onset of 0.89 V, E_1/2 of 0.79 V and an average electron transfer number of 3.72, as well as good stability and methanol tolerance. As a result, N, S-GNR-2s displayed better ORR property than either N-GNRs or N, S-GNRs, the control samples prepared with only a primary heteroatom source, strongly clarifying the significance of secondary-heteroatom-doping on enhancing the catalytic activity of carbon-based nanomaterials.

Heteroatom-Doped Porous Carbon-Based Nanostructures for Electrochemical CO2 Reduction

The continual rise of the CO2 concentration in the Earth's atmosphere is the foremost reason for environmental concerns such as global warming, ocean acidification, rising sea levels, and the extinction of various species. The electrochemical CO2 reduction (CO2RR) is a promising green and efficient approach for converting CO2 to high-value-added products such as alcohols, acids, and chemicals. Developing efficient and low-cost electrocatalysts is the main barrier to scaling up CO2RR for large-scale applications. Heteroatom-doped porous carbon-based (HA-PCs) catalysts are deemed as green, efficient, low-cost, and durable electrocatalysts for the CO2RR due to their great physiochemical and catalytic merits (i.e., great surface area, electrical conductivity, rich electrical density, active sites, inferior H2 evolution activity, tailorable structures, and chemical-physical-thermal stability). They are also easily synthesized in a high yield from inexpensive and earth-abundant resources that meet sustainability and large-scale requirements. This review emphasizes the rational synthesis of HA-PCs for the CO2RR rooting from the engineering methods of HA-PCs to the effect of mono, binary, and ternary dopants (i.e., N, S, F, or B) on the CO2RR activity and durability. The effect of CO2 on the environment and human health, in addition to the recent advances in CO2RR fundamental pathways and mechanisms, are also discussed. Finally, the evolving challenges and future perspectives on the development of heteroatom-doped porous carbon-based nanocatalysts for the CO2RR are underlined.

Magnesium lignosulfonate-derived N, S co-doped 3D flower-like hierarchically porous carbon as an advanced metal-free electrocatalyst towards oxygen reduction reaction

The development of metal-free electrocatalytic materials that are economical, friendly to the environment, and efficiency towards the oxygen reduction reaction (ORR) is of significant interest. Hence, this paper synthesizes nitrogen and sulfur co-doped three-dimensional magnesium lignosulfonate (MLS-derived) flower-like hierarchical porous carbon (NSLPC) materials by a simple and green method. The synthesized NSLPC uses magnesium lignosulfonate as the sulfur source and carbon precursor, melamine as nitrogen source, MgO as hard template, and ZnCl₂ as the activator. We also investigated the effect of the ratio of MgO to ZnCl₂ on the catalyst performance. When the ratio of MgO to ZnCl₂ is 10:0.5, NSLPC-1005 possesses the highest ORR activity with an enormous surface area (1752.54 m² g^-1), abundant active sites, and a hierarchical porous network structure. In alkaline media, NSLPC-1005 has an initial potential of 0.97 V, as well as an excellent half-potential of 0.86 V (vs. Hg/HgO), and an ultimate current density of 5.35 mA cm^-2. It exhibits attractive ORR performance as well as outstanding cyclic stability that are comparable to commercial Pt/C electrocatalysts. This research developed an effective approach to synthesize metal-free carbon materials with high activity and long-term durability as electrocatalysts, which have a promising application in sustainable energy conversion technology.

Yeast-Based Porous Carbon with Superior Electrochemical Properties

Biomass is a promising carbon source for supercapacitor electrode materials due to its abundant source, diversity, and low-cost. Yeast is an elliptic unicellular fungal organism that is widespread in nature. In this work, we used yeast as the carbon source and Na2SiO3 as the activator to prepare a honeycomb porous carbon with higher surface area. The yeast and Na2SiO3 were directly mixed and ground without any solvent, which is simple and characterized by large-scale application. The prepared porous carbon shows a good specific capacity of 313 F/g in 6 M KOH at a density of 0.5 A/g and an excellent rate capability of 85.9% from 0.5 to 10 A/g. The results suggest that the yeast-derived porous carbon may be a promising sustainable bio-material for the preparation of supercapacitor carbon electrode materials. This study provides an economical and practical avenue for yeast resource utilization and develops a simple approach to prepare porous carbon materials.

N/S Co-doped microporous carbon derived from PSSH-Melamine salt solution as cathode host for Lithium-Selenium batteries

Selenium cathode attracts great attention due to its high theoretical volumetric capacity and better electrical conductivity than sulfur cathode. Herein, N/S co-doped microporous carbon (NS-K-PC) is designed and prepared as Se host by a spray drying process of the poly(styrenesulfonic acid)-melamine salt solution followed by carbonization and activation process. The as-prepared NS-K-PC shows a very high micropore contribution of 94.8% in the total surface area, and a total N/S heteroatom doping level of 2.5 wt% in the carbon matrix. The NS-K-PC/Se cathode delivers a high reversible capacity of 499.2 mA h g^-1 at 0.1C, superior rate capacity of 324 mA h g^-1 at 8C, and great cycling stability with a capacity decay of 0.081% per cycle over 500 cycles at 1C. Additionally, a comparative study demonstrates that NS-K-PC/Se cathode with the carbonate-based electrolytes exhibit better cycling stability than those with ether-based electrolytes primarily resulted from a direct solid-solid conversion of Se to Li₂Se bypassing the formation of soluble polyselenides.

Alkaline Metal Oxide Assisting the Ionothermal Method for Efficient Fe-NX/C Catalyst Preparation

Owing to low cost and high efficiency, nonprecious metal catalysts have been widely used in various types of fuel cells. To obtain a high-activity electrocatalyst, a simple method for the synthesis of iron-modified covalent triazine frameworks by the direct heating of a mixture of FeCl3, ZnCl2, ZnO, and m-phthalodinitrile is reported. The role and a possible evolution pathway of the oxygen of metallic oxides are well discussed. To further verify our assumption, the Fe3O4 microspherical nanomaterials were synthesized and the relative Fe-based catalyst (Fe-NX/C) was successfully obtained by the ionothermal polymerization method. Fe-NX/C exhibits an extraordinary oxygen reduction reaction (ORR) performance in acidic solution, with a half-wave potential of only 25 mV negative shifts compared with Pt/C, while the power density is approximately 56% of that of Pt/C catalysts under the proton exchange membrane fuel cell testing condition. This work represents a new strategy to synthesize high-performance Fe-based catalysts toward ORR.

Metal-Free Bifunctional Ordered Mesoporous Carbon for Reversible Zn-CO2 Batteries

The fabrication of Zn-CO₂ batteries is a promising technique for CO₂ fixation and energy storage. Herein, nitrogen-doped ordered mesoporous carbon (NOMC) is adopted as a bifunctional metal-free electrocatalyst for CO₂ reduction and oxygen evolution reaction in the near-neutral electrolyte. The ordered mesoporous structures and abundant N-dopings of NOMC facilitate the accessibility and utilization of the active sites, which endow NOMC with excellent electrocatalysis performance and outstanding stability. Especially, a nearly 100% CO Faradaic efficiency is achieved at an ultralow overpotential of 360 mV for CO₂ reduction. When constructed as an aqueous rechargeable Zn-CO₂ battery using NOMC as the cathode, it yields a high peak power density of 0.71 mW cm^-2 , a good cyclability of 300 cycles, and excellent energy efficiency of 52.8% at 1.0 mA cm^-2 .

Synergistic Modulation of Carbon-Based, Precious-Metal-Free Electrocatalysts for Oxygen Reduction Reaction

Developing alternatives to noble-metal-based catalysts toward the oxygen reduction reaction (ORR) process plays a key role in the application of low-temperature fuel cells. Carbon-based, precious-metal-free electrocatalysts are of great interest due to their low cost, abundant sources, active catalytic performance, and long-term stability. They are also supposed to feature intrinsically high activity and highly dense catalytic sites along with their sufficient exposure, high conductivity, and high chemical stability, as well as effective mass transfer pathways. In this Review, we focus on carbon-based, precious-metal-free nanocatalysts with synergistic modulation of active-site species and their exposure, mass transfer, and charge transport during the electrochemical process. With this knowledge, perspectives on synergistic modulation strategies are proposed to push forward the development of Pt-free ORR catalysts and the wide application of fuel cells.